%--------------------------------------------
% 图注应为  转换机制
% 其他部分为文字和公式
%--------------------------------------------
\begin{tikzpicture}[->,>=stealth',shorten >=1pt,auto,node distance=5em,semithick]
    \tikzstyle{every state}=[text=black]
    \tikzset{nearnodes/.style={node distance=2em}}

    \node[state] (s) {s};
    \node[state] (f) [right of=s] {f};
    \node[nearnodes] (+) [left of=s] {+};
    \node[nearnodes] (-) [right of=f] {-};


    \path (s) edge node {e} (f);
\end{tikzpicture}